Supported metal catalysts are constituted by an active metal which can, for example, be a noble metal such as platinum or palladium deposited on a support in the form of particles.
The influence of particle size on catalyst activity has been widely debated in the literature for a large number of hydrocarbon transformation reactions.
For a certain number of reactions, such as hydrogenation of ethylene, propylene, cyclopentene, or benzene over supported metals such as platinum or palladium, particle size has little influence on the specific activity of the catalysts (J. C. Schlatter, M. Boudart, J. Catal. 24 (1972) 482; Y. Hadj Romdhane, B. Bellamy, V. De Gouveia, A. Masson, M. Che, Appl. Surf. Sci., 173 (1986), 383; M. Boudart, W. C. Cheng, J. Catal., 106 (1987), 134; J. M. Basset, G. Dalmai-Imelik, M. Primet, R. Martin, J. Catal., 37 (1975), 22).
The specific activity corresponds to the activity of the catalyst reduced to the number of metal atoms accessible to the molecules to be transformed. The number of accessible metal atoms can be determined by techniques involving chemisorption of probe molecules (oxygen, hydrogen, carbon monoxide) or from the particle size determined by electron microscopy. These different techniques are very well known to the skilled person.
The specific activity is also known as the turn over frequency (TOF). For such reactions and with the aim of increasing the overall activity of a catalyst for a given quantity of metal, it is important to disperse the metal on the support to the best extent, and thus to use a catalyst with the smallest possible particles.
The situation is substantially different for other reactions. As an example, for the hydrogenation of conjugated dioletins or alkynes, it has been shown that the specific activity of palladium based catalysts increases as the particle size increases. In that case, the optimum activity of the catalyst occurs when the particle size is about 4 nm (J. P. Boitiaux, J. Cosyns, S. Vasudevan, Appl. Catal., 6 (1983), 41).